In a place shared by many people, such as a public facility, a technique of providing information only to specific people is significantly useful.
For example, in many cases, the station staff desires to provide different information to a person waiting for an outbound train on a platform of a train and a person waiting for an inbound train. In addition, many people use a bank, and the communication at the reception desk and the like is often related to personal information. Therefore, it is desirable that the communication be not heard from far away.
Thus, a technique called spot reproduction that allows only people in a specific area to hear the reproduced voice is developed and actually used.
For example, a flat speaker, a parametric speaker that modulates an ultrasonic wave to generate sound in an audible range, and the like are used in a platform of a station or the like. The speakers can use high directivity to propagate sound only in a specific direction, and the sound can be delivered only to listeners in a specific direction. However, in the method, the attenuation is small in the specific direction, and the sound is transmitted far away.
In this regard, there is a method in the spot reproduction technique, in which spot reproduction is realized with respect to the distance and the direction from the speaker. This is a method of generating a wave front called an evanescent wave that is significantly quickly attenuated compared to a spherical wave.
The evanescent wave is a wave generated under a condition that the wavelength becomes shorter than the wavelength of a normal propagating wave for some reason. A method based on a combination of a speaker array and signal processing is proposed as a method of generating the evanescent wave (for example, see PTL 1 to PTL 3).
Specifically, for example, in a case of using a linear speaker array to generate an evanescent wave for sound of 1 kHz (wavelength of 34 cm), phase differences can be set stepwise between all speaker units included in the linear speaker array, and the interval of rotation (2Π) of the phase can be set to a length smaller than 34 cm.